The mainstream semiconductor devices of integrated circuits, particularly very large scale integrated circuits, are Metal-Oxide-Semiconductor Field-Effect Transistors (MOS transistors). With the constant advancement of integrated circuit fabrication technology, the semiconductor device technology node continues to reduce and the transistor geometrical size continues to shrink following the Moore's law. When the transistor size is reduced to a certain extent, a variety of secondary effects due to the physical limits of the transistors may occur one after another. It becomes increasingly difficult to downscale the transistor feature size (or critical dimension). Among many challenges in transistor and semiconductor device fabrication, the most difficult problem is often the transistor leakage current. The rising of the transistor leakage current is mainly caused by the continuously-shrunk thickness of the conventional gate dielectric layer.
The current approach to overcome the leakage current problem is to use high-k gate dielectric material, instead of using more conventional silicon dioxide gate dielectric material, and to use a metallic material for the gate electrode in order to avoid the Fermi level pinning and the boron osmotic effect when the high-k gate dielectric material and the conventional gate electrode material make a contact. The use of such high-k metal gate (HKMG) reduces transistor leakage current.
Although the high-k metal gate structure reduces transistor leakage current to some extent, the electrical performance of such transistors still needs to be improved due to the uncertainties during conventional formation process of transistors.